feat(M12): CLI entry point with dry-run, log-level, redacted error display

Implement the CLI entry point for pronote-sync:

cli/main.py:
- main() entry point with --dry-run (tri-state: None defers to settings,
  True overrides) and --log-level (choices: DEBUG/INFO/WARNING/ERROR/CRITICAL)
- setup_logging called before settings load (to capture config errors),
  then reconfigured with settings.app.log_level
- PipelineRunner.from_settings() as composition root, runner.run()
- Return codes: 0 success, 1 failure, 2 argparse rejection
- _safe_traceback: strips exception messages, replaces with "erreur expurgée",
  walks __cause__/__context__ with cycle protection
- _settings_secrets: collects redaction_secrets() + usernames + JID/recipient
- All error messages redacted via redact_secrets() with configured secrets
- DEBUG-level traceback only shown when DEBUG is enabled

cli/__init__.py:
- Module docstring added (French, Sphinx/reST)

tests/e2e/test_cli.py (8 tests):
- Dry-run and log-level propagation to composition root
- Configured dry-run preserved (tri-state None)
- Success with warnings returns 0
- Pipeline error redaction at DEBUG (sentinel secret)
- Configuration failure redacted traceback at DEBUG
- Pronote username non-disclosure
- Unexpected pipeline exception: redacted traceback at DEBUG, no traceback at INFO
- Argparse rejection of unknown log level (exit code 2)

Coverage: cli/ 94.74%, 627 total tests pass.

Co-authored-by: opencode/coder <coder@agents.invalid>
Co-authored-by: opencode/test-engineer <test-engineer@agents.invalid>

TODO.md

@@ -241,10 +241,10 @@ Composer et orchestrer toutes les étapes avec gestion d'erreurs dégradée et m
 
 Exposer le lancement du pipeline via une interface en ligne de commande.
 
-- [ ] Créer `cli/main.py` : `main()` (point d'entrée `pronote-sync`), args `--dry-run`, `--log-level`.
-- [ ] Initialiser les logs (`setup_logging`) et charger `settings` au démarrage.
-- [ ] Construire la composition root et lancer `PipelineRunner.run()`.
-- [ ] Gérer le code de retour et l'affichage des erreurs (redactées).
+- [x] Créer `cli/main.py` : `main()` (point d'entrée `pronote-sync`), args `--dry-run`, `--log-level`.
+- [x] Initialiser les logs (`setup_logging`) et charger `settings` au démarrage.
+- [x] Construire la composition root et lancer `PipelineRunner.run()`.
+- [x] Gérer le code de retour et l'affichage des erreurs (redactées).
 
 ### Critères d'acceptation
 - `pronote-sync --dry-run --log-level DEBUG` s'exécute sans effet de bord.